1. Field of the Invention
This invention relates to a manhole extension assembly and is more particularly concerned with extension rings for raising the effective height of a top ring of a manhole.
2. Description of the Prior Art
The prior art for some time has recognized the need for manhold extender means to raise the level of the cast iron manhole cover to a new pavement level when streets are resurfaced. Commonly, a metal extender ring of the necessary thickness is welded to the manhole top ring to raise the level of the manhole cover, as required. Other prior art proposals include adjustable manhole extenders, such as screw-threaded extender rings. All of these porposals will accomplish the desired result, but generally speaking they are costly to manufacture or require a good deal of labor for installation, particularly in the case of welded rings. The disadvantages of these prior art extender rings is that welded joints break under heavy traffic and mechanical joints loosen under vibration. Some examples of the patented prior art are shown in the following U.S. Pat. Nos: 1,639,495, 3,408,778, 1,908,909, 3,490,177, 2,903,875, 3,533,199, 3,385,011, 3,629,981, 3,968,600.
Even with the improvements of the above discussed patents, the common way today to raise the level of a manhole cover is through use of shims beneath the ground ring. This, of course, requires that the dirt around the ring be excavated to permit installation of the shims and return of the dirt after such installation.